When electromagnets are used to generate large magnetic fields it is often desirable or even necessary to shield the magnets to prevent the large magnetic fields from extending too far beyond the actual magnet. This is true, for example, in MR systems where the magnetic fields presently generated in the magnet center are in many cases two Tesla in strength. Such strong magnetic fields raise safety and other problems (see British Patent No. 1343275 which describes an active magnetic shielding method to prevent damage caused by stray magnetic fields of super-conducting magnets). For example, anybody entering within the magnetic field can seriously damage their magnetically encoded credit cards. Even more serious, however, is that anybody entering within such strong fields carrying a magnetizeable object such as a tool could have the tool pulled away. The object would then go hurtling like a missile through the air towards the center of the magnet. As a missile it could cause serious bodily harm. Another problem with strong magnetic fields that extend beyond the magnet is that they tend to raise havoc with pacemakers and other electronic devices.
To prevent the large magnetic fields from extending to a point where they can cause damage, it has been found necessary to shield the magnets. Basically, two types of shields are used: passive shield and active shields. With passive shields an electromagnet is surrounded by a material of low magnetic reluctance so that the magnetic flux lines outside the magnet (fringe field lines) tend to travel mainly within the material of low reluctance and not to extend appreciably beyond the material of low reluctance. A problem with such passive shields is that they add substantially to the weight and size of the magnetic resonance unit and further tend to upset the homogeneity of the field produced by the main field coils.
Alternatively, active magnetic shields have been used. Active shields are solenoid coils which surround the main solenoid coil and generate a magnetic field opposite in direction to the main magnetic field. The lines of force (flux) of the shielding solenoid coil cancel many of the lines of force of the main magnet and thereby decrease the external or fringe field of the main solenoid coil. A problem with actively shielded solenoid coils is that in addition to reducing the fringe field (i.e. lines of force outside the coil) of the magnet, lines of force within the magnet itself are also cancelled and thereby the main magnetic field is weakened.
In the past either:
(1) The main field coils have been designed to produce homogeneous fields and the shielding coils have also been designed to produce homogeneous fields so that the main field is homogeneous at the volume of data acquisition, or PA1 (2) Both the main and shielding coil together produce a homogeneous field in the volume of data acquisition. PA1 a cryogenic super conducting magnet arrangement contained in a dewar having an inner wall and an outer wall, PA1 a first solenoid coil within said dewar for generating a main large static magnetic field having lines of force in the center portion of the first solenoid coil extending in a first direction, PA1 a second solenoid coil within said dewar for generating a second static magnetic field with lines of force in the center portion of the first solenoid coil extending in the opposite direction to the lines of force generated by the first solenoid coil to cancel at least some of the lines of force that are external to the first solenoid coil. PA1 a first low reluctance means within said dewar for increasing flux lines in the path through said first low reluctance means between the first solenoid coil and the second solenoid coil to enable energizing the second solenoid coil with a lower current than would be used without the first low reluctance means in place between the first and the second solenoid coils, said first low reluctance means increasing the strength of the magnetic field at the center of the first solenoid coil compared to the strength of the magnetic field at the center of said first solenoid means in the absence of said first low reluctance means. PA1 a second low reluctance means for reducing flux lines external to said second solenoid coil, and PA1 said second low reluctance means comprising said outer wall of said cryogenic magnet arrangement.
The above referred to Patent Application provides a hybrid shield for the fringe magnetic fields comprised of solenoid type magnets without appreciably reducing the strength of the magnetic field in the center portion of the main solenoid coil by combining an actively shielded solenoid coil and a passive shield located between the main solenoid coil and the shielding solenoid coil.
It is an object of the present invention to improve over the hybrid shield.